It is generally well-known in the art of satellite control to provide an unsupported proof mass aboard a satellite, shielded from external non-gravitational forces so that it follows a purely gravitational orbit during operation of the satellite, and means responsive to motion of the satellite relative to the proof mass for controlling thrusters onboard the spacecraft which force the spacecraft to also follow the gravitational orbit, free from the effects of external surface forces such as solar radiation pressure and atmospheric drag. As a result, satellite position in orbit is predictable well in advance, thus significantly increasing the value of the satellite for navigational purposes, for example.
One major problem which was encountered in the use of early disturbance compensation systems of this type involved the need for very accurately determining mass attraction forces between satellite-carried components and the proof mass, in order to prevent such mass attraction forces from influencing the proof mass and thereby providing faulty operation of the system. In an effort to overcome the problem of mass attraction forces, U.S. Pat. No. 3,785,595 proposed the use of movable compensation masses or chargeable magnets (reacting with a diamagnetic proof mass), on three orthogonal axes relative to the proof mass, which could be controlled by commands from the ground tracking station, once the spacecraft was in orbit, to set up forces which counterbalance the mass attraction forces.